In recent years, there has been concern about the depletion of fossil fuel such as petroleum and coal, and renewable energy that can be sustainably utilized is increasingly expected. As one of the renewable energies, a solar cell and heat power generation which use sunlight are under development. The solar cell has problems that it requires cost for storage batteries used when the generated power (electricity) is stored and a loss occurs at the time of the power storage. On the other hand, a technique of directly converting the sunlight to a chemical substance (chemical energy) such as hydrogen (H2), carbon monoxide (CO), methanol (CH3OH), or formic acid (HCOOH) instead of converting the sunlight to electricity has been drawing attention. Storing the chemical substance converted from the sunlight in a cylinder or a tank has advantages that it requires less cost for storing the energy and further the storage loss is smaller, as compared with storing electricity converted from the sunlight in the storage battery.
As a photoelectrochemical reaction device that electrochemically converts sunlight to a chemical substance, there has been known, for example, a two-electrode type device that includes an electrode having a reduction catalyst for reducing carbon dioxide (CO2) and an electrode having an oxidation catalyst for oxidizing water (H2O), and in which these electrodes are immersed in water in which CO2 is dissolved. These electrodes are electrically connected to each other via an electric wire or the like. In the electrode having the oxidation catalyst, H2O is oxidized by light energy, whereby oxygen (½O2) is obtained and a potential is obtained. In the electrode having the reduction catalyst, by obtaining the potential from the electrode in which the oxidation reaction is caused, CO2 is reduced and formic acid (HCOOH) or the like is generated. In the two-electrode type device, since the reduction potential of CO2 is obtained by the two-stage excitation, conversion efficiency from the sunlight to the chemical energy is very low, namely, about 0.04%.
A photoelectrochemical reaction device using a stack (silicon solar cell or the like) in which a pair of electrodes sandwich a photovoltaic layer is also under investigation. In the electrode on a light irradiated side, water (2H2O) is oxidized by light energy, whereby oxygen (O2) and hydrogen ions (4H+) are obtained. In the opposite electrode, by using the hydrogen ions (4H+) generated in the electrode on the light irradiated side and a potential (e) generated in the photovoltaic layer, hydrogen (2H2) or the like is obtained as a chemical substance. Conversion efficiency from the sunlight to chemical energy in this case is about 2.5%, which is greatly lower than conversion efficiency to electric energy of the photovoltaic layer itself. One reason for this is that the potential is lost since a migration length of ions generated in the electrode on the light irradiated side to the opposite electrode is long.